1. Field of the Invention
This invention relates to a method for substantially increasing the acid catalytic activity of members of a unique class of crystalline zeolites by treatment with water under controlled conditions.
2. Description of the Prior Art
It is known in the art that the use of steam (water) and/or ammonia can be utilized to modify the activity of acid catalysts, e.g. clays, silica-aluminas and zeolites. Much of the emphasis in the field of catalyst activity modification has been directed towards reducing the activity of catalysts. For example, U.S. Pat. No. 4,016,218 teaches the reduction of catalytic activity of a class of zeolites having a silica to alumina mole ratio of at least 12 and a constraint index within the approximate range of 1 to 12 by the use of prior thermal treatment. Such prior thermal treatment includes the use of a steam atmosphere. It is also known that the use of ammonia can suppress an initially high activity catalyst.
Hydrogen zeolites of the 1 to 12 constraint index type are generally prepared from their alkyl ammonium and ammonium form precursors by calcining in an inert atmosphere, usually in nitrogen at about 1000.degree. F. The more costly nitrogen atmosphere is chosen over the cheaper heating in air to avoid temperature runaway and steam formation that is known to damage the catalyst and results in lower activity. Small samples in the laboratory can be calcined in air without significant steam damage if the temperature is controlled by a slow heat up and by allowing any steam formed to diffuse away. With this careful first calcination, hydrogen zeolites result that are free of residual nitrogen compounds and have the maximum number of acidic hydroxyl groups, which is equal to the number of framework aluminums. Samples thusly prepared are designated "fresh samples". The corresponding catalytic activity of these fresh samples is called "initial activity" and when measured by the alpha (.alpha.) test as described hereinafter, assigned the designation of ".alpha.o."
It has long been known that the catalytic activity of hydrogen zeolites can be reduced by high temperature heating and especially by steaming.
It is also known that the deactivation due to steam is pronounced at higher temperatures and longer reaction times. It is also more pronounced at higher steam pressures. Deactivation in the absence of steam, i.e., in an inert atmosphere, requires more severe conditions than steam deactivation.
Recently it has been found that the use of water can be employed to improve certain zeolite catalyst characteristics, while maintaining catalyst activity levels. U.S. Pat. Nos. 4,149,960 and 4,150,062 describe the use of about 0.5 to about 15 moles of water per mole of feedstock in order to substantially reduce the coking and aging rates of the zeolite catalysts used in the processes of these patents.
U.S. Pat. No. 3,493,519 teaches a method of using steam for the stabilization of Y-faujasite zeolite. There, a chelating agent was used after steaming to take out the excess aluminum from the zeolite. The resultant catalyst of this process is a hydrothermally stable zeolite catalyst having high hydrocarbon conversion activity.
In U.S. Pat. No. 3,546,100, it is disclosed that a rare earth exchanged zeolite hydrocracking catalyst such as zeolites X or Y can be improved with respect to its cracking activity and selectivity by using water in controlled amounts to activate catalyst cracking sites. This disclosure states that the amount of water be maintained during the process such that the water vapor partial pressure is kept at a level of about 10 to about 130 mm. water vapor.